SHIP inhibits Akt activation in B cells through regulation of Akt membrane localization

Activation of the serine/threonine kinase Akt and the regulation of its activation are recognized as critical in controlling proliferative/survival signals via many hematopoietic receptors. In B lymphocytes, the B-cell receptor (BCR)-mediated activation of Akt is attenuated by co–cross-linking of BCR with the inhibitory receptor Fc?RIIB1, and the binding of the SH2 domain-containing inositol phosphatase, SHIP, to Fc?RIIB1. Because SHIP dephosphorylates phosphatidylinositol 3,4,5-trisphosphate (PIP3) and activation of Akt requires PIP3, the destruction of this phospholipid has been proposed as the mechanism for Akt inhibition. However, upstream kinases that activate Akt, such as PDK1, also require PIP3 for activation. In this report, we addressed whether SHIP inhibits Akt directly at the level of Akt recruitment to the membrane, indirectly through PDK recruitment/phosphorylation of Akt, or both. We generated stable B-cell lines expressing a regulatable, but constitutively membrane-bound Akt that still required PDK-dependent phosphorylation for activation. Several lines of evidence suggested that activation of this membrane-targeted Akt is not inhibited by Fc?RIIB1/SHIP and that PDK is not a target for SHIP-mediated inhibition. These data demonstrate that SHIP inhibits Akt primarily through regulation of Akt membrane localization. We also observed during these studies that Fc?RIIB1/SHIP does not inhibit p70S6k activation, even though several other PIP3-dependent events were down-regulated. Because the enhanced activation of Akt in the absence of SHIP correlates with hyperproliferation in the myeloid lineage, our data have implications for SHIP and Akt-dependent regulation of proliferation in the hematopoietic lineage.